THIS INVENTION relates to a dielectric heating device. More particularly it relates to a dielectric heating device which employs microwave heating , suitable. for heating or cooking foodstuffs and suitable for, but not restricted to, use as a domestic or institutional microwave oven.
According to the invention there is provided a dielectric heating device which comprises:
a plurality of electrically conductive side walls which are electrically interconnected and arranged in series to define a heating cavity, at least one pair of adjacent side walls being inclined to each other at a corner having an included angle selected from the group consisting of acute angles and obtuse angles; and
at least one electromagnetic power source capable of emitting electromagnetic radiation suitable for dielectric heating and arranged to feed such radiation into the cavity at a frequency which varies with time and which causes the heating cavity to act as a multimode resonant heating cavity.
The selected angle will not be a right angle, the included angle being less than 180xc2x0 but not equal to 90xc2x0, and being either an acute angle or an obtuse angle.
Preferably, adjacent side edges of each adjacent pair of side walls physically abut, electrically to interconnect the pair of side walls. Instead, each adjacent pair of side walls may be electrically connected together along their adjacent side edges. Naturally, the dimensions of the cavity should be large enough, bearing in mind the wavelength of the radiation emitted by the power source, to ensure multimode resonant operation in the cavity.
The device may be in the form of an oven defining the heating cavity, the cavity being tubular in shape and the side walls extending from one end of the cavity to the other end of the cavity, the cavity having an outline in cross-section which is polygonal and which has a plurality of sides arranged in a peripherally extending series, the sides of each adjacent pair of which define between each other a corner of the cavity.
The cavity may have an outline in cross-section which has an odd number of sides, being eg that of a regular polygon, the polygon having sides all of the same length and the number of sides being selected from the values of three, five, seven, nine and eleven, preferably five or seven and more preferably five. The polygon may be a regular pentagon having five sides of equal length, the sides of each adjacent pair of which are inclined to each other at an obtuse included angle of 108xc2x0, each said side wall being a panel which is flat and planar in shape, and of rectangular outline. Instead, however, at least two sides may have lengths of different magnitudes. The outline in cross-section of the cavity will thus have sides which are substantially straight and, accordingly, each side wall panel will be substantially planar, eg rectangular in form, and all the sides of the outline will be of equal length, with all the included angles being equal in size, namely said 108xc2x0.
The ends of the cavity may be open, said open ends respectively defining an inlet to the cavity and an outlet from the cavity to permit continuous heating of a profile which is moved lengthwise through the oven. In this case, each end of the cavity may be provided with a choke having an opening therein for receiving, with a working clearance, a profile which is moved lengthwise through the cavity, each said clearance being selected to suppress emission of electromagnetic radiation, in use, from the cavity via the opening of the choke and around the profile.
When the ends of the tubular cavity are open, thereby defining an inlet and an outlet which is spaced from the inlet, so that the oven is suitable for continuous throughput from the inlet to the outlet of solids, such as an object or substance to be heated, the oven will, for example, be suitable for heating continuously extruded profiles, or the like, the choke at each end being constructed and/or configured to suppress or reduce emission of electromagnetic radiation from the cavity via said end in use.
The cavity may thus be elongated so that it is vertically elongated when it is upright to permit downward movement of an extrusion therethrough. The cavity may be provided with a plurality of microwave sources, at least some of which are adjustable as regards their microwave output, and which sources may be both vertically and circumferentially spaced from each other around a central zone in the cavity intended for receiving the moving extrusion, eg down which, in use, the extrusion can pass, when the cavity is upright.
The microwave sources may be arranged in the cavity, in said circumferentially and/or longitudinally spaced relationship, around a central path in the cavity down which the profile passes, the sources being arranged to direct microwave radiation at a said profile in said path. The microwave cavity may thus be provided by a vertically extending microwave oven having an outlet at its lower end and an extruder for extruding a profile, or an inlet, at its upper end, as the case may be.
Instead, the oven may have a pair of end walls which close opposite ends of the cavity, at least one of the walls of the oven forming at least part of a door into the cavity, to permit batch operation in the heating of at least one object placed in the oven. In this case, one of the end walls may form a floor for the cavity, the other end wall being spaced above the floor and forming a roof for the cavity, the oven having at least five side walls which are panels, each side wall panel being flat and planar in shape, and being rectangular in outline, and the door cooperating with a closable door opening leading into the cavity, the door opening being larger in area than the area of each side wall panel. The door may form at least one side wall panel of the oven, the door forming less than two side wall panels of the oven. The door may be selected, on the one hand, from a single door panel, hinged at one side of the door opening and, on the other hand, from a pair of door panels arranged side by side and hinged respectively at opposite sides of the door opening, the door having at least one hinge and each hinge extending in a direction from the one end wall of the oven towards the other end wall thereof.
When the ends of the cavity are closed, the cavity may be foreshortened in length, its length between its ends being less than its cross-sectional width, although, naturally, the length can be greater than the width.
In the usual way, as indicated above, the floor, roof and side wall panels will typically be electrically conductive and electrically interconnected, and the oven may have one or more door panels which close the door opening and which are similarly electrically conducting and electrically connected to said floor, roof and side wall panels. Conveniently, the floor is a flat floor panel optionally having a turntable mounted on its upper surface for rotating contents of the oven while they are being subjected to microwave heating, and the roof may similarly be a flat panel and may have a ventilation opening provided with a grid, and the roof panel may be parallel to the floor panel, the side wall panels being normal to the floor panel and to the roof panel.
In a particular construction, when the oven is regular-polygonal in horizontal cross-section, it may have a single microwave power source, such as a magnetron, klystron or the like, the oven including a waveguide for guiding microwave radiation from said source to microwave feed openings in the side wall panels for feeding microwave radiation from the waveguide into the interior of the cavity. The waveguide may extend, in the form of a hollow partial belt or collar, horizontally partially around the oven on the outer surfaces of the wall panels spaced, eg midway, between the floor and the roof of the oven. In this regard it will be appreciated that the side wall panels will be constituted, at least in part, by the door panel or panels but, typically, said door panel or panels will not have microwave feed openings therethrough and the waveguide will only extend around the oven over the side wall panels or parts thereof which are not constituted by the door panel or panels, so that the door panel or panels will not have the waveguide extending across their outer surface or surfaces.
It is expected that, when the cavity is regular-pentagonal in horizontal cross-section, the door panel or panels will typically form, in total and as indicated above, more than one side wall panel and less than two side wall panels of the cavity. When there are two door panels for closing the door opening, they will be located alongside each other, abutting each other at upwardly extending edges thereof, typically being hinged or pivoted along upwardly extending edges thereof adjacent opposite sides of the door opening; and when there is a single door panel this will usually be hinged or pivoted along an upwardly extending edge thereof, adjacent one side of the door opening.
It is expected that, for domestic use, the cavity will be located in a more or less rectangular housing, the housing being provided with conventional controls, eg on a control panel, for controlling microwave power input into the cavity, and with the usual power cut-out for automatically cutting off the microwave source from a power supply thereto, when the door or doors are open. Preferably, each microwave source will, as indicated above, be capable of delivering the microwave radiation over a relatively wide bandwidth, of at least 1 MHz in width, preferably at least 1.5 MHz. Microwave sources with such large bandwidths may not be readily available, but it is contemplated that they can be purpose-built for special ovens, such as institutional (restaurant, hotel, hospital, etc) microwave ovens, unless such microwave sources eventually become mass produced.
The device may be a microwave oven, each electromagnetic power source being a microwave source capable of emitting microwave radiation at a frequency of 0.3xc3x97109-1xc3x971010 Hz, eg 1-10 GHz. More particularly, each microwave source may be capable of emitting microwave radiation at a frequency of 2.4-2.5 GHz over a bandwidth having a width of at least 1 MHz. Preferably said frequency is the commercially available frequency of 2.45 GHz, 2.45 GHz being the central of nominal frequency of the microwave source, the microwave source being a magnetron, klystron, or the like, which may be more or less conventional in construction, being either domestic or industrial in type, as required.
Said sources may be adjustable as to the frequency/wavelength of the microwave radiation produced thereby, as to the intensity/amplitude or power of the microwave radiation produced thereby, as to the direction of propagation of the microwave radiation and/or as to the concentration or compression thereof into more or less discrete beams, etc. Thus, for example, the microwave cavity may be of the dynamically tuned standing-wave type, the multimode type to enable different overlapping standing waves to be generated, or a suitable hybrid type of of the aforegoing.
Thus the microwave radiation source or sources may be coupled into at least one of a dynamically tuned standing-wave applicator, multimode applicator or any other suitable microwave applicator. In a preferred embodiment for the purposes of drying and/or sintering large-diameter ceramic extrusions, a multimode cavity may be employed to generate overlapping standing waves. The microwave heat sources may be arranged in a microwave cavity which is located below an extruder so that the cavity can receive an extrusion in the form of an extruded profile as it is extruded. Preferably, the size and shape of the cavity is arranged to enclose the profile in a snug- or close fashion, ie so as to choke the ends of the cavity so as to reduce unwanted emission of microwaves therefrom in use, the profile blocking or occluding the ends of the cavity, which, as indicated above, may be provided with chokes.
For heating extruded profiles of large cross-section, microwave cavities of the multimode type have been found to be suitable. As indicated above, there may be a plurality of microwave radiation sources, and at least some said sources may be adjustable, eg as to power output (amplitude/intensity) and/or wavelength/frequency.
The microwave power source may, as indicated above, have a central or nominal frequency of about 2.45 GHz and a bandwidth capable of producing at least two different distributions of electromagnetic intensity, and preferably as many as feasible, for the selected cross-sectional geometry. For example, for a cross-section in the shape of a polygon with included angles of 108xc2x0 and successive sides in a circumferential direction of length 290 mm, 208 mm, 260 mm, 260 mm and 208 mm, (or a regular pentagonal cavity having sides of equal length in the range 200-300 mm) the cavity having an end-to-end length of 700-800 mm, eg 730 mm, typically requires, for each power source, a bandwidth of at least 1 MHz at a nominal frequency of about 2.45 GHz. A bandwidth of at least 4 MHz is preferred for effective operation at these or similar dimensions and nominal frequencies.
The microwave power source may be constructed to generate microwaves at a frequency which varies, eg automatically, in a desired fashion with time, For example, the frequency may vary over the bandwidth at a rate which depends upon the frequency of the power supplied to the electromagnetic power source, e.g. at a rate of 50 Hz.
The microwave power source may be arranged so that its frequency varies automatically over the bandwidth at said rate. Instead, the device may include control means to which the microwave power source is coupled and is responsive, the control means being operable to vary the frequency of the microwave power source over said bandwidth at said rate.
The dielectric heating device may include guide means for guiding electromagnetic radiation emitted by the power source, the guide means having an inlet for receiving electromagnetic radiation from the electromagnetic power source and an outlet for feeding the electromagnetic radiation into the heating cavity; and the device may include displacement means for displacing the outlet relative to the heating cavity. Instead or in addition the device may include a rotating antenna positioned at the outlet. When the power source is a microwave source each guide means may be a microwave waveguide, which may be of more or less conventional construction.
The displacement means may be arranged to alter the attitude of the outlet so that the direction in which the electromagnetic radiation is emitted therefrom and fed into the cavity is varied. Preferably, this attitude is varied by rotating the outlet cyclically about an axis extending lengthwise along the cavity at a rate of at least 1 revolution/second, depending on the nature and position of what is being heated in the cavity; and the outlet may be located in a fixed position while its attitude is altered.
Instead, the displacement means may be arranged to displace the outlet so that the position of feed of the electromagnetic radiation into the cavity is varied. Accordingly, the displacement means may include a track, eg an endless track, along which the guide means runs, thereby to vary said position of feed of the electromagnetic radiation, preferably at a rate of at least 1 lap along the track/second. Instead, the heating device may have an antenna operatively associated with the outlet of the guide means, the antenna being arranged to rotate about an axis extending lengthwise along the cavity and being arranged to emit microwaves in a direction inclined at an angle to this axis of rotation.
The heating device may have at least one further guide means, which is fixed, each fixed guide means having an outlet which is fixed or stationary relative to side walls of the oven, so that both the direction and the position of feed of the electromagnetic radiation emitted therefrom remain unchanged during heating.
In a particular embodiment, the guide means may be arranged so that its outlet feeds transversely into the cavity. Instead or in addition, the guide means may be arranged to feed longitudinally into the cavity, the longitudinal direction of the cavity being that from one end of the cavity to the other.
In the embodiment which includes at least one further guide means which is fixed, a dedicated microwave generator may be coupled to each guide means. Instead, an output of a single microwave generator may be fed to each guide means.
Preferably, the power source is thus arranged to feed microwave radiation into the cavity in a fashion so as to encourage the cavity to act as a multimode resonant cavity.
The oven may include biassing means for changing, eg continuously, the resonant field distribution in the cavity, for example a movable antenna which in use projects the microwaves into the cavity.
The invention makes possible a method of dielectric heating, using a dielectric heating device as described above, the method comprising generating electromagnetic radiation by means of each said power source and feeding the radiation into the heating cavity, the method including emitting, from each power source, radiation whose frequency varies.
The method may include varying the direction of feed of the radiation into the cavity. The method may include altering the attitude of the outlet to vary the inclination of said direction of feed of electromagnetic radiation into the cavity. Additionally or instead, the method may include displacing the outlet thereby varying the position of feed of the electromagnetic radiation into the cavity.
The invention will now be described, by way of a specific, non-limiting illustrative example.
A specific microwave cavity according to the invention, when in an upright attitude, is of regular pentagonal horizontal cross-section, having five vertically extending wall panels, each of which is provided with a vertically extending row of vertically spaced openings, each opening being provided for receiving a microwave source in the form of a magnetron/waveguide/fan assembly. The cavity has twenty such assemblies located in selected openings in the wall panels of the cavity. There are substantially more than twenty said openings, so that, by selecting suitable openings to receive the assemblies, the assemblies can be installed in said wall panels in a desired vertically and/or circumferentially spaced array, the array in turn being selected to allow the assemblies to heat an extruded profile descending through the cavity, so that it is heated at a desired rate and has a desired temperature profile along its length.
The cavity is vertically elongated, and is housed in a matching pentagonal housing, radially spaced from the cavity, made of a frame having doors attached thereto, which can be opened for easy access to the wall panels of the cavity. These doors are earth-strapped and provide an effective Faraday cage. The top and the bottom of the cavity are open.
In use, when a profile is extruded downwardly along the interior of the cavity, the space between the extruded profile and the inner surfaces of the wall panels of the cavity contains thermal insulation. A refrigerated air source is provided, together with a fan, arranged to feed a forced draft of refrigerated air upwardly into the space between the cavity and the housing, which forms an air plenum, and the fan of each magnetron assembly has an inlet communicating with this plenum. The lowermost portions of the wall panels of the cavity are water-cooled by means of a water-jacket or water-tubes (cooling coils). The cavity is about 5 m in height, and its top is provided with a fume extraction fan to remove air from the plenum. Provision is made for the controlled flow of gas of a controlled temperature and humidity in the space between the extrusion and the wall panels of the cavity.
Two optical sensors are provided in the cavity, for measuring the temperature of an extruded profile in the cavity; and three spaced thermocouples are provided in the insulation of one or more of the cavity wall panels, for monitoring the temperature profile of the extruded profile in the cavity. Each magnetron assembly has an over-temperature cut-out switch, similar switches being installed in said air plenum and/or on the cavity wall panels. Amplified diode assemblies are provided in the plenum to monitor microwave radiation continuously. Similar diode assemblies are provided at selected points external to the installation to monitor microwave radiation.